One close to home, seriously (2,7)
I believe the answer is:
in earnest
'seriously' is the definition.
(synonyms)
'one close to home' is the wordplay.
'one' becomes 'I' (Roman numeral).
'close to' becomes 'near' (close to means nearby).
'home' becomes 'nest' (a bird's home).
'i'+'near'+'nest'='IN EARNEST'
(Other definitions for in earnest that I've seen before include "seriously" , "Not joking" , "Purposefully" , "resolutely" , "Sincere and serious" .)